The manufacture of cast iron parts requires the use of certain additives known as inoculants which are incorporated into the molten iron bath during the melting and/or pouring process to obtain the desired metallographic structure and to ensure the internal health of the parts.
Inoculation is defined as the supply to a metal bath in the moment prior to the pouring of certain alloys in order to cause changes in the distribution of the graphite, improvements in the mechanical characteristics and the reduction of the tendency for whitening.
The purpose of the inoculation is the generation of germination nuclei on which the solid phases grow during solidification.
In certain cases, these seeds result from the addition of fine particles of the same phase to be solidified. These particles are not completely dissolved, giving rise to the growth of crystals. Thus, for example, the addition of graphitic carbon to a cast iron in the moment prior to the pouring promotes the nucleation of the graphite in the metal bath and prevents undercooling during solidification. However, the carbon used as additive must have a high degree of crystallization to generate nucleation seeds which enable the precipitation of the carbon in graphitic form.
This same effect can be obtained from particles of materials different from those of solidification. The increase of the number of nuclei in the molten metal favors that eutectic solidification, and especially graphitic precipitation, can take place with a minimal undercooling, which reduces the tendency for the formation of eutectic carbides and favor the precipitation of graphite. Most of the inoculants used today contain from 45 to 75% Si and variable percentages of Ca and Al mainly (the pure Si alloys are not effective in inoculation). Depending on the nature of the characteristics of the parts to be manufactured and available manufacturing processes, they can incorporate variable amounts of other elements such as Ca, Ba, Mg, Mn and Zr which are used to increase the solubility and/or the strength of the inoculant.
The inoculation can be carried out inside or outside the mold. The traditional process for external inoculation, and the most common one, consists of adding inoculant in the metal stream coming from the transfer of treatment ladle during the filling of the pouring ladle. The intention is to obtain a homogeneous mixture and a good dilution of the inoculant. This process has considerable limitations which affect both the weight of metal to be treated (it is not valid for small amounts) and the useful pouring time (the fading of the inoculating effect is very quick).
In the inoculation outside the mold, materials which are granulated or in the form of wire which are incorporated to the molten metal in various ways and at different points of the pouring line are used.
Patent GB 2069898 describes a process for wire inoculation for a pressure pouring furnace, wherein the inoculant material is incorporated to the passage of molten metal in the outlet runner of the tank, leading the molten metal to the pouring spout, at the opposite end of which is the pouring nozzle through which the mold is filled. As is inferred from the design set forth, this process has several operative defects or limitations, mainly derived from the regularity of the pouring flow. It is evident that a stop in the molding line causes the corresponding stop in the pouring unit, with the subsequent fading of the inoculating effect and the rapid cooling of the metal exposed in the open spout.
A way to prevent the mentioned problem consists of projecting inoculant particles on the pouring stream in the exact moment in which the latter enters the mold. An inoculation process of this type is described in patent JP 55122652. In this case, the drawback of the operation translates into an irregular and generally low yield, due to the loss of material occurring because of the projection itself and because of the rebound of part of the particles on the metal stream. These projection methods have an added drawback which is the difficulty in adapting the flow rate to the metal flow rate due to the fact that it occurs in the precise moment of the filling. The usual practice consists of establishing a fixed inoculant flow rate according to the average pouring flow rate, taking into account that while a mold is filled, the flow rate can range between hundreds of grams and several kilos per second. During a conventional mold filling operation, it is evident that there is a lack of proportionality, i.e., that there will be over-inoculated parts compared to other under-inoculated parts in the mold, which can give rise to defects of a contrary nature in the same mold.
In relation to the aforementioned inoculation with graphitic carbon, it can be emphasized that C has in the Fe—C diagram a saturation at the eutectic point (TE=1153° C.) of 4.26%. The alloying elements increase or decrease the temperature of this saturation point. In the inoculation with graphite, the solubility must be carefully observed. As soon as the graphitic carbon supplied dissolves, it loses its properties as a germinator, which involves a quick fading of its effect in an uncontrolled manner according to the temperature, chemical composition and degree of stirring of the hotmelt. This makes the inoculation with graphite be a little used process.
This inoculation can be indispensable in extreme conditions of the casting, such as perished metals, with low O2 content, which cause a weak reaction to the germination with oxides. In this case the incorporation of the graphite must be carried out right before filling the mold, which involved a low temperature and short waiting time for the solidification.
The appearance on the market of pouring furnaces with an inductor and pressurized with nitrogen involved a great improvement in the manufacturing processes and translated into an immediate increase of productivity. However, the quality and the manufacturing costs did not benefit equally since the new furnaces introduced new specific problems derived from their own conception and design.
These furnaces allow maintaining the metal available for the pouring for more time since the two main drawbacks mentioned above, i.e., the loss of temperature of the metal and the fading of the magnesium (in nodular cast iron) are corrected. However, it has a very important general operation problem: the furnace must always be maintained with molten metal covering the inductor, therefore the latter must always be running. The loss of metallurgical quality experienced by the metal during its recirculation through the inductor must be added to the costs derived from the maintenance of the metal during non-operative periods. It has been verified that the main parameters for controlling he cooling curve (temperature of the eutectic and recalescence) experience a progressive linear degradation according to the temperature of the metal and the dwell time in the tank.
Two already mentioned techniques are used to compensate and correct this deterioration: the metal is first inoculated during the filling of the furnace by means of supplying the material to the stream of the transfer ladle; the metal is then inoculated in the pouring stream by projection in the moment in which the mold is filled. The combination of these two techniques allows an acceptable degree of control over the metallurgical quality and is currently the commonly used process in castings which have this type of furnace.
However, the sum of negative aspects i.e., the process accumulates the defect of the fading and that of the lack of proportionality and efficiency of the inoculant, is counterposed to the sum of positive aspects. The defect of generation of slag occurring due to the supply of solid alloying agents in the pouring phase must be added to this.
Therefore, there is still a need in the state of the art to provide a new inoculation process for inoculating a cast iron which at least partly overcomes the mentioned drawbacks.